Modern aircraft employ landing navigation systems to assist a pilot in maintaining an aircraft along a predetermined glide path associated with a particular landing strip or runway. Commercial aircraft commonly incorporate an Instrument Landing System (ILS) or a Microwave Landing System (MLS). ILS systems suffer from several problems, including RF interference from nearby FM broadcasting stations and guidance beam distortions due to increased development in airport environs. Economic concerns have limited the acceptance of MLS systems.
A Global Positioning System (GPS) employs spaced-apart satellites in circular orbits at locations that are readily available to the public. One reference for satellite information is the Interavia Space Directory (1990-1991), available from James Information Group, which contains the nominal orbital locations of GPS satellites. GPS satellites continuously broadcast signals that may be received by anyone with the proper equipment. The GPS satellite signals contain ephemeris data that precisely describes the orbits of the satellites. A GPS operates on the principle of multilateration, wherein a GPS receiver makes range measurements based on the GPS signals generated by multiple satellites. The range from a satellite is determined by measuring the satellite signal transmission and receiving time in conjunction with a clock synchronized to the satellite's clock, and calculating the distance from a specified position of the satellite at the transmission time. The specified satellite position is obtained from the broadcast ephemeris data. The intersection of the multiple range measurements made simultaneously is used to ascertain latitude, longitude, and altitude. Range measurements from at least three sources are necessary to ascertain a position in three-dimensional space.
Each range measurement contains an error called an offset bias, created by the unsynchronized operation of the satellite and user clocks. This error will yield an erroneous range measurement, making it appear that the user is either closer to or farther from each of the satellites, as compared with the true distance. These measurements are therefore more accurately termed pseudoranges.
An additional error is introduced by the content of the GPS satellite transmission itself. Each GPS satellite transmits an encrypted signal for military use and a degraded, unencrypted signal for civilian use. The unencrypted signal used by commercial aircraft may introduce errors from zero meters to 100 meters or more. Physical factors also introduce uncertainty in range calculations. Atmospheric propagation delays and multipath interference are two such major factors.
One technique for reducing the effects of the error in a GPS utilizes differential corrections for the pseudoranges measured by a GPS receiver to eliminate common errors, namely, offset biases. Differential corrections can be determined by placing a GPS ground station receiver at a precisely known, fixed reference site, and determining pseudoranges to GPS satellites. Actual errors are then determined by comparing the calculated pseudoranges with the values expected for the known reference site. The differences between the received and expected values are then transmitted to the GPS receiver over a separate datalink to enable the receiver to correct pseudorange measurements before the position of the receiver is computed.
A fixed differential GPS (DGPS) ground station used in an aircraft landing environment typically includes a datalink signal transmitter for transmitting GPS correction data and approach data associated with a particular landing strip. Approach data includes the identity of the approach and coordinates that sufficiently describe the desired flight path for the approach to the landing strip.
Current systems offering DGPS for private use and commercial aircraft are built to a Requirements and Technical Consideration for Aeronautics (RTCA) standard known as "Special Category I" (SCAT I), which is documented in RTCA document DO-217. The SCAT I system uses a datalink situated in the VHF navigation band (112 to 118 MHz). The physical layer uses a differentially encoded eight-phase shift keyed (D8PSK) waveform. The media access sublayer uses time division multiple access (TDMA) with a slot structure maintained by synchronization with GPS time.
One shortcoming of SCAT I is that it lacks precisely specified performance standards of interfaces necessary to support interoperability. This makes it difficult for avionics manufacturers to build equipment to a standard such that the equipment will work with any ground station equipment. Another shortcoming of the SCAT I system is the relatively crowded nature of the VHF navigation band. In some areas of the world, this band is heavily used. Additional frequencies may not be available within the VHF navigation band for use in a DGPS. Thus, a need exists for an improved method of transmitting datalink signals to an aircraft, while minimizing interference caused by heavy use of the VHF navigation band.
DGPSs that use of one or more pseudolites to augment satellite GPSs have been proposed for implementation in aircraft landing systems. A pseudolite consists of a ground-based station with a transmitter that transmits signals similar to those transmitted by a GPS satellite. An aircraft can combine the range measurements calculated from pseudolite signals with satellite range measurements to further reduce errors. The use of a pseudolite in addition to a differential ground station increases the costs of building and maintaining a DGPS system. In summary, a need exists for an improved, economical method of providing range measurements to an aircraft during landing. This invention is directed to fulfilling this need.